The DXF Reference is online available at Autodesk.

Quoted from the original DXF 12 Reference which is not available on the web:

Since the AutoCAD drawing database (.dwg file) is written in a compact format that changes significantly as new features are added to AutoCAD, we do not document its format and do not recommend that you attempt to write programs to read it directly. To assist in interchanging drawings between AutoCAD and other programs, a Drawing Interchange file format (DXF) has been defined. All implementations of AutoCAD accept this format and are able to convert it to and from their internal drawing file representation.